Volume range expander circuit



` Dec 22, 1942-, P. F.' G. HoLsT VOLUME RANGE EXPANDER CIRCUIT Filed Aug; 5l, 193? type of volume control fore.

' phonograph reproducing For phonograph record operation, or

` weak or low average PatentedkDec. 22, 1942 UNITED, STATES r'rsNrf-osilcs vownmnasotszmnncmcnrn I I.

Paul F. G. Holst, Oaklyn, NJ., assigner to Mo Gorporation of America, a .corpcration' of .Delaware Y l Application august s1, ma, sei-uu N9. v292.789

1 claim. (ci. 11s-m) The present invention -relates to-volume range expander circuitsy for audio frequency amplifiers.

= and has for its primaryobiect `to provide an imv proved volume range expander circuit of the t character referred to, which may be adjusted to operate over a useful portion of a predetermined volume range of expansionfor various signal levels with a given ampliflen'andjwhichmay be turned'oif and on without objectionable change in signal volume or soundoutput level from a4 sound producing device connected with the controll'ed ampliner.

In the usual volume range expanderi circuit lfor electric phonograph amplifiers, a. potentiometer device is provided to control the amplitude of signals applied tothe 'expander circuit. It

. essary and may'even A culties inthe adjustment has been-found that Aa maior portion of the operating movement or control range of such potentiometer devices provides no enective control of the expander action. Thereit is a further object of to provide a'volume range expander circuit which is not subject to this undesirable operating condition land which gives substantially full control of therange of dynamic expansion over the fullA strong .2o the present invention range movement of) the control element of the circuit for a range of-signal levels ordinarily en- -countered in audio frequency 'signal conveying )circuits and ampliners.

It is also an object of thel present invention to provide avolume range expander circuit which s operates in response to variations in the'average amplitude of applied signals to'provide volume range expansion eectively in connection with systems, and which may be adjusted to respond' within a limited useful control range to variations in the amplitudelof l recorded signals on phonograph records.

ation in response to any signals which may vary in amplitude, the selected ranges -of expander control :may vbe considered. for example; with vregardto the expander` circuit response to `(i) a strong or relatively high amplitude passage on a loud or high average amplitude'record; (2) -a weak or relatively low amplitude passage-on a loud -or high average amplitude record; (3)v a loud or relatively high amplitude passage on a weaker low averagev amplitude record; and i4) a weak or relatively low amplitude passage on a amplitude record. i The same consideration of signal strength `,%variation,for expander operation may be given in connection with any'audio` frequency .amplifier having a variable amplitude signalsupply.

' withthis type'circuit,

of 'the' present invention to provideA a for oper- I vrange expander control circuit wherein substanunder item 1 above.

'but not off, and

effective control control device or input the volume range expander circuit.

accordance with .variations in the average .a relatively light lo'ad is rechner. whereby directv The maximum' suchvthat this type of signalwill cause full expansion within the limits of-operationof the expander circuit, pander to maximum.

that is, "-will operr up the ex- Additional gain is unneccontrol circuit operative in laccordance with vthe foregoing conditions, whereby full volume range expansion action may be obtained on .all types `phonograph records 'with a minimum of inaction in the operation of a potentiometer means' for In a volume range expander circuit arranged in accordance with the produce a controlling voltage which varies in amplitude of the said applied signal. In connection it is also a further object volume tially all of the voltage output of the diode rectiffier is made available for control purposes through an improved connection and coupling arrangement'for'the diode rectifier, and wherein provided by the diode circuit of a ampliner tube may be utilized inthe expander system. The'invention will, howeverrbe better under.

` stood from the following description when considered in connection with the 'accompanying drawing. and its scope appended claim.

Inthedrwln t Figure 1 is a schematic circuit diagram'of an audio frequency amplifier -system for phonograph vgain required' from the ex pander amplifier under item 3 .above Ishould be be harmful and, as pro- A for which conditionof operinvention, as in-known circuits of this nature. a diode rectifier device isemployed for rectifying the vappliedsi'gnalsto coupling to the output is pointed out* in the thus far described is representative expander control circuit embodying the invention,y b

.Figure 2 is-a schematic circuit diagram ,ofa portion of the circuit offFig. 1 with certain of record reproduction provided with a volume range the circuit elements and'connectio'ns rearrangedv 'whereby the .operation of the circuit of Fig.

may be more readily apparent, and l Figure 3 is a graph showing curves illustrative of 'an' operating characteristic ofthe system of Fig. l, I

Referring to Fig. 1. the amplier system shown by way of example comprises a source of audio' frequency signal'such as an electric phonograph pickup device 5 connected through an adjustable control network 6 and an output circuit compriso ing a'high signal potenti/al lead 9 and a low signal able bias source such as a self-bias resistor 44, as indicated. The output anode 45 of the ampliiler'tube is provided with an anode coupling resistor 46 and coupling capacitor 41 through which amplied signal energy'is appliedtoa `diode rectiiler 48 in such a manner that the diode 48 and an output lresistor 5II therefor are con nectedl eil'ectively in parallel with the ampliiier output resistor 46 through the coupling capacitor 41.

By inverting the the liode anode 5I and one terminal 52, oi' the output resistor 50, to ground as indicatedat"59,'4

while the cathode 54' is vconnected tothe high signalpotentlal terminal 55 ofthe output resistor 50 through a lead 55,-substantially all o! the D. C. potential resultingjrom signal rectifipotential lead 9 to an output volume control device IU-I I. The lead 9 of the phonograph-pick- `up output circuit is grounded, as indicated, to

the chassis and to the metallic motor'board or other frame member of the phonograph mechanism indicated by the' dotted enclosure I2.

The ampliiier system further includes a combined amplifier andY volume expander tube I5 having a signal input grid I6 coupled to the volume control output contact I I through a coupling capacitor I1 and having a bias supply resistor I9 'connected back to a tap I9 between two sections` 2li and '2| of a self-bias resistor. The latter is connected between the cathode 22 and chassis or ground 23 which is also the negative side oi the anode potential supply system. The positive lead the anode supply system'is indicated at 24. The output anode of the tube is indicated at 25 and is coupled in the usual manner to a succeedingampliiier represented by the rectangle 26,

cuit. The `connection and a'sound producing or output device such as a` loudspeaker, indicated at 21, connected to the output circuit firing system. r

The combined ampliiler and volume expander tube I5 is preferably of the type having a `separate gain controlling grid indicated at 30 to which vthe latter being 26 of the amplivariable gain controlling potentials with respect to cathode,I are lapplied through the" delay iilter 4b toot u.' the grid potential .may above the value oi' the potential proved sensitivity,

cation may be developed across Ithe single resistor and may be applied to the control-cir- 4' cuit 33. y

This arrangement provides a highly eii'ective and eillcient voltage supply circuit having an imf while being greatly simplined with respect to the circuit and circuit components required, over rthe usual diode rectiiler cir-1 between the diode cathode diode, that is, by connecting 54 andthe supply lead 33 is provided through a series iilter resistor indicated at 51.

Since the diode anode 5I is connected toround n and therefore to the ground :fand to the negative end of the self-bias potential sourceV 20-2I of the ampliiler tube I5, the increasing positive output potential developed atthe terminal 55 appearing across the input circuit 9-9 from the "'signal source -trol grid 30 oi' the ampliier 5, causes the potential of the con- I5to be raised in a positive direction above ground.v f the adjustment ofthe potentiometer inputv conlbe raised to or resistors 2 5 and 2I as an initial the grid 30.

Since theconnections are negative biasl on such that the potential across the diodeoutput resistor 59 is in oppor' p sition to the bias potential provided by the source comprising a series resistor 3| andz a shunt cav This tube, for exand a suppressor.'

2li-2|, the effect of increased signal amplitude is to increase the positive bias'betweengrid.

type known commercially includes a potentiometer device connected across the signalinput circuit 8`9 in shunt thereto and. comprises three resistor or impedance sections 35,

36 and 91 in Aseries between the, high and lovvsidesl of the circuit, the center section 36 having an adjustable output tap which is connected to pander amplifier andy through a lead 46 on the connection 38 thereon aL separate volume exdetector channel `39 high potentialside and l.chassis or ground connection on the low potential side.` The purpose and relative values of resistor sections will hereinafter be discussed.

The 'expander ampliiler and detector channel preferably includes a buier amplifier tube- 4I.

having a signal input orcontrol grid 42 connectedto the inputlead 4I) and having a cathode` 43 connected to chassis or ground through a suitand cathode 22 in opposition to the negative with respectl to ground,-in response to signals Depending upon provided -by the biasing potential .betweerr the cathode 22 and ground 23, that is, to reduce the initial negative bias potential, and if permitted to increasel sumciently high, the grid 30 may be driven up to a positive' range of operation.

The operation of the lcircuit -will be further understood from a consideration of Fig. 3

wherein the curves 63, 64 and 55, drawn with logarithmic1 coordinates, represent input-output characteristicsiory diil'erent fired values ci gain in the amplifier. The curve 64 represents the highest permissible gain and curve 63 represents the lowest gain in the` operatingrange of the amplifier. `,'Il1e various intermediate values oi gain between the two limits represented by the I curves53 and 64 may be obtained by adjustment of -the biasing potential applied to the fcontrol grid :n of the-ampnnor ls. 1 Since the control channel operates in response to signals from the main signal channel, thej control voltage applied to theA electrode 3| increases with .an increase in 'the signall input voltage. The corresponding values of the inputoutput characteristic will consequently be located in a curve corresponding to different ampliiier gains and-the oscillating input-output curve .will

' asoaevs appear as shown at 60. The signal input below the value indicated by theline 6I does not produce suiicient control voltage to ailect the gain of .the amplifier. while signalv voltages higher than those indicated by the line 82 serve to hold the-gain at its maximum value by reason of the grid. current fiow,` ashereinafter pointed out. y

A high degree of volume expansion is obtained between a certain initial biasing potential'value and zero biascprres'pondingv to a point in the operation when the rectiiled signal bias equalsV the initial bias. This is represented by the curve S between the limits of signal input at the dotted lines 6| and 62, the initial biasing potential being such that the'ampliflcation is substantially constant up to the signal input-'value indicated by the line 8 I At signal input voltage values higher than those represented by the line 62, no appreciable increase in amplification is secured, since the control grid 30'reaches zero 'bias for an inputl signal amplitude corresponding to the line 62..

' position of the In order to avoid the disadvantages encountered with either method of operation referred to, with a given minimum and maximum condition of gain established, for example as indicated at 03 and 64 in Fig. 3. a ilxed bias condition for the expander amplifier is so chosen for the"oi! expander control," the amplication or gain lies substantially midway between the minimum and maximum values oi the expansion range asy indicated by the line 65 in Fig. 3. To this. end, in the amplifier shown, a switch I1 is provided in connection with the expander control element 38 and connected therewith as indicated by the dotted connection 68 to close the switch B1 when the contact 38 engagesthe con'- tact 66 in the expander ofi"'position. the

switch controls a circuit cmprising a lead 60 connected to the amplier cathode 22 and a leadV A 10 connected through aseries resistor 1I to a In passing from one condition' to the other, the

ampliilcation changes from a minimum indicated by the lineA resented bythe -line the zero biasrange, resistors 3l, 51 and gain level 63 to amaximum level rep' 6,4. When the tubeenters the series resistance of the 50 cause the resulting grid as a limiting control means in the circuit between vthe grid and the cathode.

' 'In a given amplifier for phonograph or radio operation, signal input levels above a certain high value represented by the input level at the vline 62 are unnecessary and provide a relativelyslight amount of volume expansion, while belovi1 a certain lowJ level of input amplitude represented by the line el, the volume expansion is relatively slight. The adjustable control elementv 36-38 `in the .input circuit of the volume expander may beadjusted -to provide an `applied signal anda rectified control potentialfor operating the ampliiier forl volume'expansion, that is, in the range represented between the lines 6| and 62 oLFig. 3.

It has been found that not "all types of audiov frequency signals or recordings arel adapted for volume range expansion. Therefore. itis desirable to providea position of adjustment .for thel control means 35-38 at which the expander may' vbe rendered inoperative. For thisl purpose,A the -controlarm-38 is arranged to contact the low atedvat its lowest gain as indicatedby the line 63 of Fig'. 3. Inthe expander "oii position, the gain may be increased to the maximum value indicated by the line 64 in Fig. 3 by means oi a' connection such that substantially zero bias is lapplied to the gain control grid 30. .However,

both arrangements have certain disadvantages.

minimum gain causesY the amplier to have a relatively` low output on weak signals or recordings, while the second has the disadvantage that at. maximum gain willv cause the `out operating put to increase abruptlyrthroughthe entire exturned off, causing objectionable 'noise in the output device. v

' current to provide a relatively high potential drop tially two-thirds of the way down 'on the potenl `tial source 2li-2|, andas this source comprises in the range most eiiective point 12 on the resistor network 3|'. l1, 5l he tween the resistors 3l and 51.

bypass capacitor 1I is provided for theA bias source 2li- 2| as indicated at 14. ,x

Referring to Fig. 2 along with Fig. l, the circuit arrangement of Fig, 1 is rearranged and' simplifled, whereby it may be seen that when the switch 6191s closed, the gain control grid 30 is connected to a tap point whichis th terminal 12 on a potentiometer resistor comprising the re sisters 1I, 51 and 5|! across the intiai bias potential source 20-2 I. Since the resistor 1l may'have a value oi substantially 2.7 megohms and the re- 'sistors 3|, 51 and 50 may have a resistance of 560,000 ohms each, the tap point'12 is substana variable resistor at 2|, the potential may be adjusted to any suitable'valueto bring the expander controlled amplier gain substantially midway of its -minimum and maximum values, when the expander is turned oi'. Y

With this arrangement, the expander may be cut into into operation or turned of! as desired" during the playing oi a record or during a radio program without appreciable change in the v'gain 'Y of the amplier and without noticeable and, un-y 51) time, by making the input signal control means Y "uxiliary ground `contact 6B, thereby insuring'that no controlsignal is supdesirable abrupt change in volume.- At the same "-30 for the expander eirectl've over a limited r/ange by the limitingf resistors I5 and l1, to permit' the' expander .to respond in an operating range between signal loads corresponding to This point is provided with a filter capacitor 13 to ground. A

strong or relatively high `amplitude passages on high average amplitude and low average amplitude records or radio programs, the iull operatl ing range. f the expander control may be uti' made, thisy would result in the -amy. plifier in the expander oilf position being operlized. This 'is providedvby the series controlling circuit comprising the nxed resistors 35 and 31 which render the expander ineffective over the initial bias and zero bias control ranges, while the.'control element 38 is eiective to provide full or extended control for proper expanderA action 'over the range therebetween.

.The'flrst has the disadvantage that'ope'rating at Kpanded range when theV expander control is` pander amplifier channel by adj potentiometer contact 38 is dete Y or the average amplitude level of With the expander ampliiler in operation, the average signal famplitude to be applied to the exent of the ined by the) average signal strength o! the received signal played. y

The range ot'control provided 'by the potentiometer element "-18 is such that with the contact adjusted for minimum signal input to the expander channel. that is, with the contact the record being 3l at the lower end or the resistor element 3l asl.

viewed in the drawing, for minimum dynamic expansion, the relative resistance values of the potentiometer drop producing elements 35, 38 and v31 are such that volume range expansion action takes place in response to signals oi relatively high amplitude above a normal range of high $305,873 1 v tion of a volume rangeexpandercircuit.- variable signal input potentiometer means for said circuit,V i a signal ampliiier tube having van, input electrode connected with said potentiometer means, 'a diode average amplitude signals or in response to relal v tivelyhigh amplitude passagesl in loud or high average amplitude records.

When the volume expander control device is adjusted with the contact 38 at the upper endmediate average signal amplitude values, the exe" rectifier coupled to said tube and having a cathode operative at a high signal potentialwhereby said diode is inverted, an output resistor for said diode rectiiier, a' combined ampliiler and volume expander'tube for Jsaid ampliflerhaving a gaincontrolgrid and a-cathode, self-bias means in circuit with thel cathode providing a potential sourcel for establishing a predetermined condif ing grid current limiting resistors, potential dropeter means serially on each side thereof for ex' tending the feiIective control rangenfor volume'- range expansion substantially over the full varia.-v l tion range thereof and for establishing'a predeor low average amplitude record. For `intr-A pander control device is adjusted with the contact I8 at suitable intermediate positions as required. l

- By limiting thev expander"y action to signals above a predetermined relativelyl low amplitude and below a predtermined relatively high amplitude', the range of control of the device 36"-38 is extended to cover the full range of 'movement provided by the device in its ntrol range and overloadingV and distortion are prevented.

I claim as my invention: l' In an audioi'requency amplier, the combination of low gain for said last-named ampliner tube. means Afor applying to said grid a biasing potential from said source and said idode rectif iler in opposition, saidlast namedmeans'in'cludproducing means in circuit with said potentiomtermined condition of maximum gain for signal input voltages ranging'a-bove a predetermined low 'peakl amplitude and below a predetermined high average amplitude, and means for establishing on said gain-control grid a biasing potential eiiectiveto set the ilerat a level substantially midwayl between said limiting resistors.

maximum and minimum v gaim-comprising va switch connection betweenV said. cathode and a tap intermediate between two of) said gridA current l 'PAUL F. G; Horan gain oi said signal ampli- 

